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Security Concepts

Different Layers of Security

How much security do you need? The only way to obtain a fully secure system is to disconnect it from the network, from all removable media devices, and from the printer, remove all I/O ports, and lock it in a secured room with a posted guard. Some computer systems warrant such security, and get it. However, that level of security isn't convenient for the normal computing needs of today's enterprises. Absolute security tends to clash with ease of use.

So what does each level of security entail? What is the right mix of levels for you and your userbase? These next few paragraphs will attempt to help answer those questions.

No Security

Some people do this intentionally on certain systems, known as honeypots, that are part of an Intrusion Detection System. This is not the way to run a corporate network in today's Internet environment. Not for any length of time anyway.

In addition to opening yourself up to data theft, data loss, and general mayhem, any system misused to attack other systems opens possible exposure to liability. Your legal department probably has some suggestions for you on that front.

Hardened Security

Hardening is about minimizing risk and increasing robustness. Hardening a server or workstation that is directly Internet accessible includes the following:

  • Reducing the number of network services that are running and accessible to the Internet.

  • Removing unnecessary software and features, which reduces the overall complexity of the system. This includes removing additional protocols such as IPX, AppleTalk, NetBIOS, DLC, LAT, and DecNET.

  • Removing software that allows access to internal system information, such as SNMP.

  • Removing insecure remote control software, such as X Window or the ADMIN$ share for remote Windows NT administration.

  • Applying all known security updates and service packs. Often times security exploits in necessary software such as DNS are resolved in a simple configuration change or security patch.

  • Enabling traffic filters if available. Windows NT and most UNIX systems have built-in or freely available software that allows more control of the kinds and sources of IP traffic that are accepted.

  • Removing unnecessary accounts on the system. Rename existing administrative or root accounts to something unique and hard to guess.

  • Removing unnecessary and overly generous file permissions for both accounts and the file system.

Hardening a server or workstation costs nothing more than the time invested, and perhaps a little research for documents. So there is no additional cost of hardware or software licenses.

Many operating system vendors have knowledge-base articles or How-To documents describing step-by-step how to harden the operating systems and common services such as Web servers. Any capable system administrator should be able to follow those documents. If not, you might need to find new support staff.

Although hardened systems might still be vulnerable to newly discovered system exploits, those vulnerabilities can be plugged.


A firewall is a device or a group of devices that enforce your corporate access control policy on two or more connected networks. Enforcing the access control policy either permits or denies traffic based on various rules you define.

When installing a firewall, you must first have a clear idea of what services you want to expose to the Internet, and what services your userbase requires access to. The policy on access to those services directly determines which rules are implemented on the firewall.

The firewall generally deals with the direction of traffic in relation to the external or internal view. For example, the normal default policy for external traffic inbound to your network is to be blocked. The default policy for internal traffic outbound to the Internet is to be permitted.

When configuring the firewall, you will have to add and modify those rules to match your policy. You should also validate your firewall after making changes to the rule base to ensure that a mistake does not expose your network to possible attack.

Although most people think of a firewall as a dedicated computer running some add-on software, there are many variations of firewalls. Most routers today have embedded firewall software, although most likely you will need to purchase a license to enable those features. Additionally, there is a new breed of network appliances that offer both the Internet connectivity of a router, and the security and performance of a firewall. The correct device, or combination of devices, depends both on your needs and your budget.

Keep in mind that firewalls are not magic pixie dust that you can sprinkle at the entry point to your network and make it all secure. A firewall does not normally perform functions such as scan email for viruses. Although there are add-on products for some commercial firewalls to perform that kind of function, it's good policy to keep things simple. Adding additional functionality creates the opportunity for that functionality to be exploited.

A firewall connecting your network to the Internet does not protect your internal servers from internal attacks.

Finally, a Web or DNS server that has not been sufficiently hardened can still be compromised through a firewall, because you are still allowing external traffic through to that server.

Demilitarized Zone

A Demilitarized Zone (DMZ), as it relates to network security, is a collection of hardened servers, firewalls, routers, and networks that isolate your internal network from the Internet.

The networks exposed on the DMZ are known as Bastion LANs. Bastion LANs are local area networks that are tightly regulated as to what traffic is allowed on, or off, of their segments. A Bastion LAN acts as a buffer between your internal network and the Internet. The Bastion LAN is usually connected to your internal network via packet-screening routers or firewalls. The hardened servers are designed to offer up the necessary exposed services to the Internet, such as an external view of your corporate DNS, externally visible Web server, mail relay, and so on.

DMZs allow you to architect redundancy, scalability, and flexibility into your network connectivity. Even though the DMZ layers of security are not sufficient individually, when combined, those layers hopefully provide a comprehensive strategy and solution to Internet connectivity and security.

Intrusion Detection Systems

Intrusion Detection Systems (IDS) are mechanisms that are put in place to detect when your system is under attack and to take appropriate action. That action is definable by your corporate policy, and could range from doing nothing, to logging the events for later analysis, to proactively creating rules on firewalls to attempt to stop the attack.

There are several components to Intrusion Detection Systems, not all of which need to be implemented to be effective:

  • Network Intrusion Detection Systems—These sniff packets on the wire to look for suspicious network traffic patterns and attack signatures. NIDS can be agents that run on individual servers, or dedicated systems that monitor network segments of hosts. Packet capture and analysis is CPU intensive, and NIDS that monitor whole network segments are not very useful in high traffic areas.

  • System Integrity Verifiers—These are programs put into place to validate the integrity of system files on exposed systems. Usually a database is created containing the name of the file with its calculated hash value. This database is usually stored on read-only media such as write-protected floppy disks or a CD-ROM; that way the attacker cannot update the system integrity database.

  • Log File Monitors—These monitor event logs on Windows NT systems or syslog files on UNIX systems for anomalous events.

  • Decoys or Honeypots—These are systems specifically left vulnerable to attack so that those attacks can be monitored, logged, and analyzed. It's nice to keep people out, but a more proactive means of security is to know who's knocking on what, and when. There are software packages, deception toolkits, available that assist you in the forensic analysis of the attack. Some of these are Network Associates' Cybercop Sting, NETSEC's Spectre, Network Flight Recorder's BackOfficer Friendly, and Fred Cohen's Deception Toolkit. We'll talk about them in greater detail in Chapter 10 "Implementing Security."

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